Automatic composition of composite images or video with stereo foreground objects

ABSTRACT

A processing device generates composite images from a sequence of images. The composite images may be used as frames of video. A foreground/background segmentation is performed at selected frames to extract a plurality of foreground object images depicting a foreground object at different locations as it moves across a scene. The foreground object images are stored to a foreground object list. The foreground object images in the foreground object list are overlaid onto subsequent video frames that follow the respective frames from which they were extracted, thereby generating a composite video.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/233,882 filed on Sep. 28, 2015, the content of which is incorporatedby reference herein.

BACKGROUND Technical Field

This disclosure relates to video or image processing, and morespecifically, to automatically generating composite images and videosfrom a single video or image sequence.

Description of the Related Art

A composite image is formed by superimposing components of two or moreseparate images or image segments into a single image. For example, whenan object moves across a background scene, a composite image may showsnapshots of the object at various time points in its motion overlaidover the background scene. Composite images therefore provide aninformative and interesting depiction of an object's path in a singleimage.

Composite images may also be used as individual frames of video to show,for example, a trail of an object being generated as the object movesacross a scene. However, conventional techniques for generatingcomposite video frames are computationally inefficient and producevideos with undesirable visual artifacts.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The disclosed embodiments have other advantages and features which willbe more readily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates an example embodiment of a media processing system.

FIG. 2A illustrates an example embodiment of a composite image.

FIG. 2B illustrates an example embodiment of video frames includingcomposite frames.

FIG. 3 illustrates an example embodiment of an application forgenerating composite images or video frames.

FIG. 4 illustrates a first example embodiment of a process forgenerating a composite image from a sequence of input images.

FIG. 5A illustrates an example embodiment of a process for reducingnoise in a composite image.

FIG. 5B illustrates example intermediate images in a process forreducing noise in a composite image.

FIG. 6 illustrates a second example embodiment of a process forgenerating a composite image from a sequence of input images.

FIG. 7A illustrates a first example embodiment of a composite image withuniform spacing between foreground object images.

FIG. 7B illustrates a second example embodiment of a composite imagewith uniform spacing between foreground object images.

FIG. 8 illustrates a third example embodiment of a process forgenerating a composite image from a sequence of input images.

FIG. 9 illustrates an embodiment of a process for generating a compositeimage from a sequence of input images when the camera is experiencingcamera motion.

FIG. 10 illustrates an embodiment of a process for generating acomposite image having stereo foreground objects from a sequence ofstereo input images.

DETAILED DESCRIPTION

The figures and the following description relate to preferredembodiments by way of illustration only. It should be noted that fromthe following discussion, alternative embodiments of the structures andmethods disclosed herein will be readily recognized as viablealternatives that may be employed without departing from the principlesof what is claimed.

Reference will now be made in detail to several embodiments, examples ofwhich are illustrated in the accompanying figures. It is noted thatwherever practicable similar or like reference numbers may be used inthe figures and may indicate similar or like functionality. The figuresdepict embodiments of the disclosed system (or method) for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles described herein.

Configuration Overview

In a first embodiment a composite output video is generated from aninput video having a sequence of frames. A current video frame isreceived for processing from the sequence of frames and a processingdevice determines whether the current video frame meets first criteria.Responsive to the current video frame meeting the first criteria, theprocessing device performs a foreground/background segmentation based ona predictive model to extract a foreground object image from the currentvideo frame. Here, the foreground object image comprises arepresentation of a foreground object depicted in the current videoframe with background pixels subtracted. The foreground object image isstored to a foreground object list that stores a plurality of previouslyextracted foreground object images. Each of the foreground object imagesin the foreground object list are overlaid onto the current video frameto generate a composite video frame. Beneficially, theforeground/background segmentation can be performed only at frames fromwhich the foreground object images are extracted and need not beperformed at every frame. Furthermore, background motion in the outputvideo is preserved.

In a second embodiment, a foreground object list comprising a pluralityof previously extracted foreground object images is stored, in whicheach of the foreground object images comprising a representation of aforeground object with background pixels subtracted. A current videoframe is received for processing from the sequence of frames and aforeground/background segmentation is performed based on a predictivemodel to extract a foreground object image for the current video frame.A processing device determines if the foreground object image meetsfirst predefined criteria. Responsive to the foreground object imagemeeting the first predefined criteria, the foreground object image forthe current frame is stored to the foreground object list. Each of theforeground object images in the foreground object list is then overlaidonto the current video frame to generate a composite video frame.

In a third embodiment, a range of frames is selected for processing fromthe sequence of frames and a foreground/background segmentation isperformed on each of the frames in the range of frames to extract aplurality of candidate foreground object images based on a predictivemodel. The candidate foreground object images each comprise arepresentation of a foreground object depicted in a corresponding videoframe with background pixels subtracted. Based on an image metric, aselected foreground object image is selected from the plurality ofcandidate foreground object images. The selected foreground object imageis stored to a foreground object list. The foreground object images inthe foreground object list are then overlaid on a current video frame togenerate a composite video frame.

In a fourth embodiment, a sequence of image frames depicting aforeground object are received where the sequence of image frames arecaptured by a camera experiencing motion. For selected frames in thesequence of image frames, foreground/background segmentations areperformed to extract respective foreground object images each comprisinga representation of the foreground object with background pixelssubtracted. The respective foreground object images are stored to aforeground object list. A respective motion of the camera is determinedfor each of the respective foreground object images in the foregroundobject list between the camera capturing the selected framecorresponding to the respective foreground object image and the cameracapturing a current frame. The representations of the foreground objectsin the foreground object images are transformed based on the respectivemotions to generate transformed foreground object images. Thetransformed foreground object images are overlaid onto the current frameto generate a composite output image.

In a fifth embodiment, a sequence of stereo video frames depicting aforeground object is received. For selected frames in the sequence ofstereo video frames, foreground/background segmentations are performedto extract respective stereo foreground object images each comprising arepresentation of the foreground object with background pixelssubtracted. The respective stereo foreground object images are stored toa foreground object list with each of the stereo foreground objectimages having left and right images with a disparity between them. Thestereo foreground object images are transformed to adjust the respectivedisparities between the respective left and right images based on achange between a convergence depth for the respective selected framesand a convergence depth for the current frame. The transformed stereoforeground object images are overlaid onto the current frame to generatea composite output image.

Media Processing System

FIG. 1 is a block diagram of a media content system 100, according toone embodiment. The media content system 100 includes a network 120, acamera 130, a client device 135 and a video server 140. In alternativeconfigurations, different and/or additional components may be includedin the media content system 100.

The camera 130 can include a camera body, one or more a camera lenses,various indicators on the camera body (such as LEDs, displays, and thelike), various input mechanisms (such as buttons, switches, andtouch-screen mechanisms), and electronics (e.g., imaging electronics,power electronics, metadata sensors, etc.) internal to the camera bodyfor capturing images via the one or more lenses and/or performing otherfunctions. In one embodiment, the camera 130 is capable of capturingspherical or substantially spherical content. In other embodiments, thecamera 130 may capture images or video having a non-spherical wide anglefield of view or a standard field of view.

The video server 140 receives and stores videos and/or images capturedby the camera 130. Furthermore, in one embodiment, the video server 140provides the user with an interface, such as a web page or nativeapplication installed on the client device 135, to interact with and/oredit the stored videos and to generate output videos relevant to aparticular user from one or more stored videos. The videos stored by thevideo server 140 may include traditional videos having, for example, 30frames per second or 60 frames per second, or videos formed from asequence of burst-captured images or time-lapsed images.

In a burst mode, for example, the camera 130 may capture a given numberof frames (burst of photos) over a given time window. In someimplementations, number of photos per burst may be configured by theuser, e.g., between 1 and 1000. In some implementations, the time windowduration may be user selected (e.g., between 0.1 s and 10 s) ordynamically configured by the camera given user preferences (e.g.,inter-shot duration), detected user activity (e.g., sky diving, surfing,biking), available storage, image resolution, bracketing configuration(e.g., 3 exposures per shot), and/or other settings. By way of anillustration, a skateboarder, attempting to record a jump, may configurethe camera to collect a burst of 30 frames within 1 s time window.

When operating in a time lapse mode, the camera 130 may be configured tocapture one or more images at a given interval. The capture may commencebased on an indication by the user (e.g., press of the record button,voice command, camera shake, clap, and/or other indication). In someimplementations, the time lapse image acquisition may be initiatedautomatically by the camera based on a given condition (e.g., timerexpiration, parameter breaching a threshold (e.g., ambient lightreaching a given level during pre-dawn/dawn), arrival of a wirelesscommunication (e.g., text message, ping), and/or other condition). Thetime lapse photo acquisition interval may be configured, for example,between 0.1 s and 120 s. In some implementations of time lapse photoacquisition, the camera 130 may be configured to take a single image(photo) at the specified interval or a plurality of images (e.g.,2-100). Multiple images may be utilized, e.g., when bracketing forexposure and/or focus distance. Duration of the time lapse may beconfigured by the user.

A user can interact with interfaces provided by the video server 140 viathe client device 235 in order to edit or view the videos and images.The client device 135 is any computing device capable of receiving userinputs as well as transmitting and/or receiving data via the network120. In one embodiment, the client device 135 is a conventional computersystem, such as a desktop or a laptop computer. Alternatively, theclient device 135 may be a device having computer functionality, such asa personal digital assistant (PDA), a mobile telephone, a smartphone oranother suitable device. The user can use the client device 135 to viewand interact with or edit videos stored on the video server 140. Forexample, the user can view web pages including video summaries for a setof videos captured by the camera 130 via a web browser on the clientdevice 135. Alternatively, the editing and viewing interfaces describedherein may execute locally on the client device 135 without necessarilyrequiring the video server 140.

One or more input devices associated with the client device 135 receiveinput from the user. For example, the client device 135 can include atouch-sensitive display, a keyboard, a trackpad, a mouse, a voicerecognition system, and the like. In some embodiments, the client device135 can access video from the camera 130, and can transfer the accessedvideo to the video server 240. While FIG. 1 shows a single client device135, in various embodiments, any number of client devices 135 maycommunicate with the video server 140.

The network 120 enables communication between the video server 140, theclient device 135, and the camera 130. The network 120 may include anycombination of local area and/or wide area networks, using both wiredand/or wireless communication systems. In one embodiment, the network120 uses standard communications technologies and/or protocols.

Various components of the environment 100 of FIG. 1 such as the camera130, video server 140, and client device 125 can include one or moreprocessors and a non-transitory computer-readable storage medium storinginstructions therein that when executed cause the processor to carry outthe functions attributed to the respective devices described herein.Furthermore, the processes described herein may be performed in thecamera 130, on the video server 140, or on the client device 125.

Generating Composite Images or Video

In an embodiment, an application executing on the camera 130, the videoserver 140, or the client device 125 receives an input video comprisinga sequence of frames and generates a composite image or a video havingcomposite video frames. FIG. 2A illustrates an example of a compositeimage 200. In the composite image 200, two foreground object images FG₁,FG₂ have been extracted from selected earlier video frames and overlaidon a current video frame, thus depicting the foreground object at itscurrent location and a subset of prior locations. This achieves thevisual effect of showing the historical path of the object on a singleimage.

Composite images may be used as frames of an output video. Here, theforeground object images FG₁ and FG₂ are overlaid onto each frame ofvideo that follows the frame from which they were extracted. As thevideo progresses, additional foreground object images may be extractedat selected frames and then added to all subsequent frames together withFG₁ and FG₂. Thus, each frame of video depicts the foreground object ata number of prior locations so as to have the visual effect of“freezing” a copy of the object at selected locations along itshistorical path.

FIG. 2B illustrates sample frames from an output video having compositeframes. A first frame 222 depicts a frame of video prior to anycomposite frames being generated. At a later frame 224 a firstforeground object image FG1 has been overlaid onto the scene. At anothersubsequence frame 226 a second foreground object image FG2 has beenoverlaid onto the scene in addition to foreground object image FG1, thuscreating the visual effect of showing a trail of the foreground object'spath. Apart from adding the overlaid foreground object images FG1, FG2,the video frames are otherwise preserved. For example, background motionis preserved as can be seen, for example, by the background object BGwhich moves across the scene as the frames advance.

FIG. 3 illustrates an embodiment of an application 300 for generating acomposite image or a video having composite frames from an input imagesequence 302 (e.g., a video). In one embodiment, the application 300comprises a segmentation engine 310, an object overlay engine 320, aforeground object list 330, and a predictive model 340. The segmentationengine 310 and object overlay engine 320 may be embodied as instructionsstored to a non-transitory computer-readable storage medium that whenexecuted by a processor causes the processor to carry out the functionsattributed to these components as described herein. Furthermore, theforeground object list 330 and predictive model 340 may be embodied asdata structures stored to a memory (e.g., a non-transitorycomputer-readable storage medium).

The segmentation engine 310 receives the input image sequence 302 andfor one or more images of the input image sequence 302 performs aforeground/background segmentation to separate a foreground object imagefrom a background image. The foreground object image depicts theforeground object with the background pixels subtracted out (i.e., thepixel locations corresponding to the background in the foreground objectimage are transparent). The segmentation engine 310 performs theforeground/background segmentation based on a predictive model 340described in further detail below. The foreground object typicallyexhibits some motion relative to the background image and thus appearsat different spatial locations in different images of the input imagesequence 302. At each image in which the segmentation engine 310performs the foreground/background segmentation, the extractedforeground object image is stored to the foreground object list 330. Theforeground object images in the foreground objects list 230 thereforerepresents a sequence of foreground object images as extracted fromvarious frames in the input image sequence 302.

The object overlay engine 320 performs image overlays to generatecomposite images (which may be frames of video). Particularly, togenerate a composite image, the object overlay engine 330 overlays eachof the previously stored foreground object images in the foregroundobject list 330 onto a base image to generate a composite image. Thisoverlay replaces the pixels of the base image with pixels of each of theforeground object images at pixel locations corresponding to theforeground object in the foreground object images. An output imagesequence 304 is generated by overlaying the foreground object imagesonto one or more images in the input image sequence 302.

In an embodiment, the segmentation engine 310 performs segmentationusing an adaptive Gaussian mixture model (GMM). A GMM is a probabilisticmodel represented as a weighted sum of K Gaussian component densities

$\begin{matrix}{{{P\left( X_{t} \right)} = {\sum\limits_{i = 1}^{K}{\omega_{i,t}{\left( {{X_{t}\mu_{i,t}},\Sigma_{i,t}} \right)}}}},} & (1)\end{matrix}$

where X is a K-dimensional data vector, ω_(i), i=1, . . . , K are themixture weights, and

(X_(t)|μ_(i,t), Σ_(i,t)), i=1, . . . , K are the Gaussian componentdensities.

has the form

$\begin{matrix}{{\left( {{X_{t}\mu_{i,t}},\Sigma_{i,t}} \right)} = {\frac{1}{\left( {2\; \pi} \right)^{K/2}}\frac{1}{{\Sigma_{i,t}}^{1/2}}{\exp \left( {{- \frac{1}{2}}\left( {X_{t} - \mu_{i,t}} \right)^{T}{\overset{- 1}{\sum\limits_{i,t}}\left( {X_{t} - \mu_{i,t}} \right)}} \right)}}} & (2)\end{matrix}$

with mean vector μ_(i,t) of the i^(th) Gaussian mixture at time t andcovariance matrix Σ_(i,t) of the i^(th) Gaussian mixture at time t. Themixture weights satisfy the condition Σ_(i=1) ^(K)ω₁=1.

GMMs are often used for data clustering since the specific distributionof the data is not required to be known a priori. In an adaptive GMM,the number of components within the model are adjusted as new datapoints are received, allowing the model to be more robust to thevariability in the data. In the training step used herein, a pixel-wiseapproach is taken in which an adaptive GMM is applied to every pixel.For example, beginning with the first image I₀ in the sequence, eachpixel's red-green-blue (RGB) color values are clustered using the GMM.The clustered pixels generally may represent the same object or similarcolors in the image. The number of clusters is variable which providesadaptability to varying scene changes such as illumination changes. Aproperty of the adaptive GMM is that foreground objects in an image areusually represented by clusters with small weights. Therefore, thebackground image can be approximated by the B largest clusters:

{circle around (p)}(x|X,BG˜Σ _(i) ^(B)ω_(i)

(x; μ _(i), σ_(i) ² I))   (3)

where x is a sample pixel and χ represents all the pixel data. If theclusters are sorted according to their descending weights, then

B=argmin_(b)(Σ_(i=1) ^(B)ω_(i,t) >M)   (4)

where M is a measure of the maximum amount of data that can belong to aforeground object without influencing the background image. Theremaining clusters are considered to represent the foregrounddistribution. The foreground can contain multiple foreground objects (orclusters of pixels). In one embodiment, additional post-processing maybe applied to determine if the foreground pixels represent a singleobject or multiple distinct foreground objects. Equations (3) and (4)can be used to generate a binary mask representing the locations of theforeground and background objects.

Given a new frame at time t+1, each pixel undergoes a match test, whichdetermines if it belongs to a particular cluster. The Mahalanobisdistance is used to perform the match test:

((X _(t+1)−μ_(t+1))^(T)Σ_(i−1) ^(b)(X _(t+1)−μ_(t+1)))^(0.5) <k*σ _(i,t)  (5)

where k is a constant value (e.g., k=3.0). If the sample pixel passesthe match test (the computed distance is less than the threshold), thenthe sample pixel belongs to that Gaussian cluster. If there are no goodcluster matches, a new cluster is generated. In order to be robust todynamic changes in the scene, the parameters of the GMM are updatedusing an exponentially decaying envelope that limits the influence ofold data. If the sample pixel passes the match test for one of the KGaussian clusters, the parameters are updated as follows:

$\begin{matrix}{{w_{i + 1} = {w_{i} + {\alpha \left( {1 - w_{i}} \right)} - {\alpha \; c_{T}}}}{\mu_{i + 1} = {\mu_{i} + {\frac{\alpha}{w_{i}}\left( {X_{t} - \mu_{i}} \right)}}}{\sigma_{i + 1} = {\sigma_{i} + {\frac{\alpha}{w_{i}}\left( {{\left( {X_{t} - \mu_{i}} \right)^{T}\left( {X_{t} - \mu_{i}} \right)} - \sigma_{i}} \right)}}}} & (6)\end{matrix}$

The hyper-parameter α is the learning rate for the model, specifying howquickly the model is updated. Similarly, α can be described as theinverse of the time adaptation period T, such that α=1/T. In otherwords, T is the number of frames used within the GMM. Therefore, α isusually small (between 0 and 1) where a small alpha (0.001) leads to aslowly updating background model. Additionally, the number of Gaussianclusters is selected by using the Dirichlet prior, c_(T), and applyingit to the update weight equation. The Dirichlet prior c_(T) has negativeweights (hence the negative sign on αc_(T)) which suppresses theclusters that are not supported by the data. When a cluster's weightw_(i) becomes negative, it is discarded, thus adaptively adjusting thenumber of components within the GMM.

If the sample pixel fails the match test for all of the K Gaussians, theparameters are instead updated as follows:

w _(i+1)=Lowest Prior Weight

μ_(i+1) =X _(i+1)

σ_(i+1)=Large Initial Variance   (7)

Lastly, if the maximum number of clusters has been reached, then thecomponent with the smallest weight is removed.

In alternative embodiments, a different foreground/background model isused that is not necessarily based on a GMM.

FIG. 4 illustrates an embodiment of a process for generating compositeframes from a sequence of images (e.g., a video). The predictive modelis initialized 402 based on a plurality of training images usingequations (1)-(7) above. For example, in one embodiment, the trainingimages comprise the first P images of a sequence of images I_(t)=0, . .. , N−1 where N is the total number of images. The images may be eithertime-lapsed images or sequential video frames.

After training the predictive model using the P training images, acurrent image I_(t) in the image sequence I is received 404 forprocessing. It is next determined 406 whether or not to extract aforeground object image at the current image I_(t). For example, in oneembodiment, the foreground object image is extracted every Y images. Inan example implementation, Y is set to 15 images. If it is determined toextract the foreground object image at the current image, aforeground/background segmentation is performed 408 on the current imagebased on the predictive model to extract the foreground object image.For example, in one embodiment, equations (3)-(4) described above areapplied to generate the background image and the foreground object imagefrom the current image I_(t) in the form of a binary mask representingthe pixel locations of the foreground pixels. The extracted foregroundobject image is then stored 410 to the foreground object list. If it isdetermined not to extract the foreground object image at the currentimage, steps 408, 410 are skipped. Then, whether or not a segmentationwas performed for the current image, all of the foreground object imagesin the foreground object list are applied 412 to the current image I_(t)to generate the composite image. In one embodiment, the foregroundobject list may store the RGB pixels to be directly overlaid in thecomposite image. Alternatively, the foreground object list may store thebinary mask which is then applied to the corresponding frame to generatethe RGB pixel data to apply to the composite image.

The predictive model may also be updated periodically or when certainconditions are met. For the current image, it is determined 414 whetheror not to update the predictive model. For example, in one embodiment,the predictive model is updated every X images where X is typically lessthan Y. In an example implementation, X is set to 2, thus updating thepredictive model every second image. In alternative embodiments,different values for X may be used. If it is determined to update thepredictive model at the current image I_(t), the predictive model isupdated 316 according to equations (5)-(7) described above. The processthen repeats from step 404 for the next image in the sequence.

When generating a video according to the process of FIG. 4, thebackground is updated every frame. Thus, motion in the backgroundappears the same in the output video as in the input video. Furthermore,a new foreground object image at its present location is added every Yframes. The visual effect is one of showing a path of the foregroundobject as it moves across a video background scene. Beneficially, thecomputationally intensive process of performing theforeground/background segmentation does not need to be performed forevery image and is instead performed only at the images in which theforeground object is to be added to the foreground object list (e.g.,every Y images).

In one embodiment, performing the foreground/background segmentation instep 414 comprises processing the segmented images to remove or reducenoise. FIG. 5A illustrates an embodiment of a process for noise removaland FIG. 5B illustrates example images visually depicting the noiseremoval process. A binary mask I_(fgd) 553 is obtained 502 from anoriginal image 551 representing pixel locations corresponding to apreliminary foreground object image. A median filter (e.g., a 5×5 medianfilter) is applied 504 to remove speckled-like noise, thus producing anintermediate image I_(median). The median filtering removes much of thenoise in the true background areas, but it also removes some trueforeground areas as well. To replace the lost foreground pixels, afilled convex hull region I_(hull) 555 of the preliminary foregroundimage is detected 506. The foreground pixels and any extra pixelsremoved from the median filtering are added 508 by applying a bit-wiseOR of I_(median) and I_(fgd) to generate a temporary image I_(tmp).Then, only the pixels within the filled convex hull regions are retained510 while other pixels outside the filled convex hull region arediscarded. This operation may be performed by a binary AND of I_(tmp)and I_(hull) to generate I_(noisyhull) 559. Gaps in the foregroundregion are closed 512. For example, a binary morphology closingoperation (e.g., using a disk size of 9×9) may be applied toI_(noisyhull) 559 to close small gaps. Furthermore, any remaining holesof a user-specified size may be filled to generate the foreground objectimage I_(cleanfgd) 561.

In an alternative embodiment, instead of having a fixed frame interval Yfor updating the foreground object list, the foreground object list maybe updated at variable intervals based on an analysis of the images asillustrated in the embodiments of FIG. 6. This technique may be used,for example, to generate composite images with uniform spacing betweenthe foreground object images in the composite image even if theforeground object is not moving at uniform speed. Alternatively, thistechnique may be used to improve the quality of the foreground objectimages in the composite image by selecting foreground object images thatmeet some predefined image criteria.

In the process of FIG. 6, a segmentation is performed 602 on a referenceframe to generate an initial foreground object and initialize thepredictive model. A current image in the image sequence is then received604 for processing. A foreground/background segmentation is thenperformed 606 on the current image to extract a foreground object image.If an image criteria is met at step 608, the extracted foreground objectimage is added 610 to the foreground object list and otherwise step 610is skipped. For in one embodiment, an image quality metric may be usedto determine 608 whether or not to add the foreground object image tothe foreground object list. For example, an image having a sharpnessabove a particular threshold may be added to the foreground object list.In another embodiment, the presence or absence of a particular imagefeature may be used to determine 608 whether or not to add theforeground object image to the foreground object list. Here, forexample, an image may be added to the foreground object list if a faceis detected in the foreground object image. In another embodiment,motion parameters may be used to determine 608 whether or not to add theforeground object to the foreground object list. For example, the objectmay be added when it is detected that it is at its peak height, peakvelocity, peak acceleration, etc.

In one embodiment, in order to provide uniform spacing between theoverlaid foreground objects snapshots, an overlap percentage isdetermined between the foreground object extracted in the current imageand the most recently stored foreground object in the foreground objectlist. If the percentage drops below a threshold T_(d), the criteria instep 608 is met and the foreground object image for the current imageI_(t) is added to the foreground object list. In another embodiment, asecondary criteria may be used to select among a group of foregroundobject images that fall within a predefined range of overlap percentagesusing any of the selection criteria described above (e.g., imagequality, face detection, motion parameters, etc.). This ensures that thebest foreground object image will be selected from among a group ofcandidate foreground object images, that will each approximately meetthe desired spacing from the previous foreground object image.

FIGS. 7A and 7B provide examples of composite images in which each ofthe foreground object images is selected to have an approximatelyuniform overlap with the previous foreground object image. For example,in FIG. 7A, the overlap percentage threshold T_(d) is set to 3%. In FIG.7B, the overlap percentage threshold T_(d) is set to 21% thus resultingin foreground object images that are closer together than in FIG. 7A.

FIG. 8 illustrates another embodiment of a process for generating acomposite image or a video having composite image frames. In thisembodiment, rather than processing the sequence of images one frame at atime, the video is post-processed in a manner that processes a range offrames at a time to generate a plurality of candidate foreground images,and then a foreground image is selected from among the candidates to addto the foreground object list. Particularly, a range of images in thesequence of images is selected 802 for processing. For example, a groupof Z consecutive frames of a video are selected or a group of Z burstimages are selected from a time lapse image capture. Aforeground/background segmentation is performed 804 based on thepredictive model to extract a candidate foreground object image fromeach frame in the selected range. A foreground object image is thenselected from among the candidate foreground object images. For example,the foreground object image may be selected based on any of the criteriadescribed above such as, for example, the foreground object image havinga highest quality metric, a highest likelihood of face detection, a peakmotion parameter, etc. The selected foreground object image is stored808 to the foreground object list. This process may repeat foradditional frame ranges. For example, the process may repeat every Yframes where Y>Z. This ensures that at least one foreground object imageis added to the foreground object list every Y frames, but provides someflexibility in which foreground object image is selected in order toimprove quality of the output video. Furthermore, theforeground/background segmentation need not be performed for thenon-selected frames (e.g., frames that are not in the group of Zframes). After generating the foreground object list, a composite outputimage or video is generated. For example, to generate an output video,the foreground object images are selectively overlaid on appropriateframes of the output video such that for any given frame, the foregroundobject images corresponding to frames earlier than the given frame areoverlaid on the given frame. The process of FIG. 8 may additionallyinclude periodically updating the predictive model (e.g., every Xframes) as described above.

In any of the above-described methods, a transformation may be appliedto one or more foreground object images to compensate for camera motionthat occurs between capturing different images corresponding to thecomposite frames. In this manner, each composite frame depicts theforeground object from the current camera viewpoint. Thus, for example,if both the foreground object and the camera are moving from left toright such that the foreground object remains centered in each frame,the foreground object images can be transformed to compensate for thecamera movement and properly depict the left to right motion of theforeground object, thus depicting a sequence of foreground objectsbeginning near the left edge and ending at the center at the presentlocation of the foreground object. Similarly, foreground object imagesmay be scaled to compensate for the camera moving along a z-axis (i.e.,into or out of the scene), closer to or further from the path of theforeground object. In some embodiments, where multi-view images areavailable or can be generated, three-dimensional foreground objects maybe constructed that can be rotated to compensate for rotation of thecamera about the object. Thus, for example, the foreground object imagemay be rotated to provide the view of the overlaid foreground objectthat the camera would have seen had it captured the overlaid foregroundobject from its current rotational position.

FIG. 9 illustrates an embodiment of a process for generating a compositeimage in the presence of camera motion. A sequence of image framesdepicting a foreground object is received 902 that are captured by acamera while the camera is undergoing motion. As a result of the cameramotion, the actual path of the foreground object is not clearly definedfrom the image sequence. For selected frames in the sequence of imageframes, a foreground/background segmentations is performed 904 toextract respective foreground object images that each comprise arepresentation of the foreground object with the background pixelssubtracted. The process of selecting frames from which to extract theforeground object images and performing the foreground/backgroundsegmentation may be performed according to any of the variousimplementations described above or other implementations. Eachrespective foreground object image is stored 906 to a foreground objectlist. Motion information of the camera is also tracked 908 and storedcorresponding to each foreground object image. For example, the camera'smotion may be tracked based on various motion sensors integrated withthe camera, and motion or position data may be stored in associated witheach foreground object image that represents a relative or absoluteposition or motion of the camera when the corresponding frame wascaptured. Alternatively, image analysis may be performed to determinecamera motion without necessarily relying on sensor data from in-camerasensors. For example, in one embodiment, features are extracted andtracked between different frames. The feature motion may then be fit toa motion model of the camera to estimate the camera motion.

To generate a composite frame, each of the respective foreground objectimages in the foreground object list is processed 910 to determine arespective motion of the camera between the position of the camera whenthe corresponding frame was captured and the current position of thecamera corresponding to the frame on which the foreground object imagesare being overlaid. Each of the foreground object images is thentransformed 912 based on the respective motions to generate transformedforeground object images. In one embodiment, the transformation maycompensate for at least one of translational motion of the camera in x,y, and/or z directions, rotational motion of the camera about areference point (which may correspond to the foreground object), andpointing motion of the camera corresponding to a change in the cameraorientation along any of the three axes. The transformed foregroundobject images are then overlaid 914 onto the current frame to generate acomposite output image. As described above, this technique may be usedto generate videos with composite frames that have a dynamic movingbackground or this technique may be used to generate individualcomposite images from a sequence of images.

Different translations may be applied to the foreground object imagesdepending on the type of camera motion between the frame when theforeground object image was captured and the frame on which it is beingoverlaid. For example, in order to compensate for planar motion of thecamera in the x,y plane, a location of the representation of theforeground object in each of the foreground object images being overlaidis translated to compensate for the change in planar position of thecamera between capturing the selected frame corresponding to theforeground object image and capturing the current frame over which theforeground object image is being overlaid. Here, the transformation mayinvolve translating the location of the overlaid foreground object imagein a manner equal and opposite to the change in planar position of thecamera between capturing the selected frame corresponding to theforeground object and capturing the current frame.

In another example, in order to compensate for motion of the cameraalong the z-axis (e.g., towards or away from the scene), the translationinvolves scaling the representation of the foreground object in theforeground object image based on a component of the motion representinga change in position along the z-axis (e.g., depth axis) betweencapturing the selected frame and capturing the current frame.Particularly, the foreground object may be enlarged in response to thecamera being moved closer to the position of the foreground object whenthe selected frame was captured, and the foreground object may bereduced in size in response to the camera being moved further away fromthe position of the foreground object when the selected frame wascaptured.

In another example, a location of the foreground object may betranslated based on a component of the motion representing a change inrotational position of the camera about a reference point betweencapturing the selected frame and capturing the current frame.Particularly, the location may be translated according to a motion equaland opposite to the motion representing the change in rotationalposition of the camera between capturing the selected frame andcapturing the current frame. Furthermore, in embodiments in whichthree-dimensional foreground object images are available (as may becaptured, for example, by multi-view camera systems or generated fromstereo views using image processing techniques), the foreground objectimage may be rotated to compensate for the change in rotational cameraposition about the object.

In yet another example, the location of the foreground object may betranslated based on a component of the motion representing a change inpointing orientation of the camera between the current frame and theselected frame. Furthermore, when a change in pointing orientationoccurs, a lens distortion effect may be applied to the representation ofthe foreground object based on the change in pointing orientation of thecamera between the current frame and the selected frame. For example, ina wide-angle or “fisheye” lens, objects appear more stretched around theedges of the image than in the center of the image. Thus, an foregroundobject image that was originally captured in the center portion of theimage but overlaid near an edge of a subsequent image due to a change inpointing orientation may have a distortion effect applied to mimic howthe object would have looked if originally captured near the edge of theimage. Similarly, a foreground object originally captured near an edgeof an image may have a reverse distortion effect applied if it appearsnear the center of the image in the frame on which it is overlaid.

In one embodiment, a partial transparency may be applied to overlaidforeground object images that appear very close to the camera so thatthey do not block the view of the rest of the scene in a case where theobject path has a component along the z-axis towards the camera or awayfrom the camera. For example, in one embodiment, it is detected when anoverlaid foreground object image exceeds an overlap threshold with oneor more other overlaid foreground object images. A partial transparencyis then applied to the detected foreground object image exceeding theoverlap threshold. In other embodiments, different threshold criteriamay be used. For example, a partial transparency may be applieddependent on a percentage of the image covered by the overlaidforeground object image. In other embodiments, the partial transparencymay be applied dependent on a detected depth of the overlaid foregroundobject. In additional embodiments, the amount of transparency may bevariable and change with the distance of the object to the camera usingany of the detection methods discussed above.

In other embodiments, where a camera at least partially rotates aroundthe foreground object image at a rate significantly faster than motionof the object, a multi-view foreground object image may be storedcomprising a set of two or more foreground object images each depictingthe foreground object from a different viewpoint. The multi-viewforeground object image enables the foreground object to be rotatedwithin the composite frame to depict any one of the different views. Theparticular view may be selected manually by a viewer via a userinterface that enables the user to rotate a three-dimensional or partialthree-dimensional view of the foreground object image to provide aninteresting viewing effect. Alternatively, the particular view of themulti-view foreground object image may be selected automatically whenoverlaying the multi-view foreground object image on a frame. Forexample, the view may be selected that best matches the camera viewpointin the selected frame on which the multi-view foreground object image isbeing overlaid. In one embodiment, the multi-view foreground objectimage may comprise a stereo foreground object image having left andright images that can be viewed as a three-dimensional object using aconventional stereo viewer. In yet further embodiments, similar effectsmay be applied to multi-view foreground object images capturedconcurrently using multiple cameras from different viewpoints, such as,for example, a stereo camera or multi-view camera system.

In yet other embodiments, composite images or video frames may begenerated in for stereo (three-dimensional) images in which eachoverlaid foreground object image comprises a stereo image having leftand right views. The disparity between the left and right imagescorresponds to the depth of the image when viewed using a stereo viewingsystem. Particularly, foreground object images having a positivedisparity (e.g., the object location in the left image is to the left ofthe object location in the right image) appear behind a convergencedepth (e.g., corresponding to the viewing screen), foreground objectimages having a negative disparity (e.g., the object location in theleft image is to the right of the object location in the right image)appear in front of a convergence point, and foreground object imageshaving zero disparity (e.g., the object location is the same in the leftand right images) appear at the convergence depth. When overlayingstereo foreground objects on stereo frames, the disparity between theleft and right foreground object images may be adjusted to provide theappropriate disparity based on their depth and the convergence depth forthe current frame. For example, the convergence depth for the currentframe may be set to correspond to the depth of the foreground objectbased on its current location. Overlaid foreground object images behindthe plane corresponding to the current object depth are depicted with apositive disparity having increasing magnitude as they become fartheraway from the viewer while overlaid foreground object images in front ofthe plane corresponding to the current object depth are depicted withnegative polarity having increasing magnitude as they become closer tothe viewer.

FIG. 10 illustrates an embodiment of a method for generating a compositeoutput image having stereo foreground objects. A sequence of stereovideo frames are received 1002 depicting a foreground object. Forselected frames in the sequence of stereo video frames,foreground/background segmentations are performed 1004 to extractrespective stereo foreground object images which each comprising arepresentation of the foreground object with background pixelssubtracted. The respective stereo foreground object images are stored1006 to a foreground object list. Each of the stereo foreground objectimages thus include left and right images with a disparity between them.The stereo foreground object images are transformed 1008 to adjust therespective disparities between the respective left and right imagesbased on a change between a convergence depth for the respectiveselected frames and a convergence depth for the current frame. Forexample, the disparity is decreased (i.e., becomes less positive or morenegative) for a given stereo foreground object image in response to theconvergence depth for the current frame being further away than aconvergence depth for a selected frame corresponding to the given stereoforeground object, and the disparity is increased (i.e., becomes morepositive or less negative) in response to the convergence depth for thecurrent frame being closer than the convergence depth for the selectedframe corresponding to the given stereo foreground object.

In addition, any of the techniques described above for compensating forcamera motion (e.g., translations, rotation, scaling, partialtransparencies, etc.) may similarly be applied to stereo foregroundobject images. Notably, some level of rotation may be achieved whenstereo foreground object images are available and thus rotation may beapplied to account for rotational camera motion.

In alternative embodiments, instead of directly capturing stereo frames,two-dimensional frames may be captured and stereo foreground objectimages may be generated from the two-dimensional capture by artificiallyapplying a disparity to the extracted foreground object image. Wherecamera motion involves rotation about the object, views from differentorientations may be selected to create the stereo foreground objectimages so as to provide a more complete three-dimensional view of theobject.

Stereo foreground objects may be used together with any techniquesdescribed above in FIGS. 1-9 to generate composite videos havingcomposite frames and dynamically moving background or such techniquesmay be used to generate individual composite images.

Additional Configuration Considerations

Throughout this specification, some embodiments have used the expression“coupled” along with its derivatives. The term “coupled” as used hereinis not necessarily limited to two or more elements being in directphysical or electrical contact. Rather, the term “coupled” may alsoencompass two or more elements are not in direct contact with eachother, but yet still co-operate or interact with each other, or arestructured to provide a thermal conduction path between the elements.

Likewise, as used herein, the terms “comprises,” “comprising,”“includes,” “including,” “has,” “having” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus.

In addition, use of the “a” or “an” are employed to describe elementsand components of the embodiments herein. This is done merely forconvenience and to give a general sense of the invention. Thisdescription should be read to include one or at least one and thesingular also includes the plural unless it is obvious that it is meantotherwise.

Finally, as used herein any reference to “one embodiment” or “anembodiment” means that a particular element, feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. The appearances of the phrase “in oneembodiment” in various places in the specification are not necessarilyall referring to the same embodiment.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative structural and functional designs for thedescribed embodiments as disclosed from the principles herein. Thus,while particular embodiments and applications have been illustrated anddescribed, it is to be understood that the disclosed embodiments are notlimited to the precise construction and components disclosed herein.Various modifications, changes and variations, which will be apparent tothose skilled in the art, may be made in the arrangement, operation anddetails of the method and apparatus disclosed herein without departingfrom the scope defined in the appended claims.

1. A method for generating a composite output image from an input videohaving a sequence of frames, the method comprising: receiving a sequenceof stereo video frames depicting a foreground object; for selectedframes in the sequence of stereo video frames, performing by aprocessing device, foreground/background segmentations to extractrespective stereo foreground object images each comprising arepresentation of the foreground object with background pixelssubtracted and storing the respective stereo foreground object images toa foreground object list, each of the stereo foreground object imageshaving left and right images with a disparity between them; transformingthe stereo foreground object images to adjust the respective disparitiesbetween the respective left and right images based on a change between aconvergence depth for the respective selected frames and a convergencedepth for the current frame; and overlaying the transformed stereoforeground object images onto the current frame to generate a compositeoutput image.
 2. The method of claim 1, wherein transforming the stereoforeground object images comprises: decreasing the disparity for a givenstereo foreground object image in response to the convergence depth forthe current frame being further away than a convergence depth for aselected frame corresponding to the given stereo foreground object; andincreasing the disparity for the given stereo foreground object image inresponse to the convergence depth for the current frame being closerthan the convergence depth for the selected frame corresponding to thegiven stereo foreground object.
 3. The method of claim 1, furthercomprising: setting the convergence depth for the current frame based ona depth of a current position of the foreground object in the currentframe relative to a current camera position when the current frame iscaptured.
 4. The method of claim 1, wherein transforming the stereoforeground object images further comprises: scaling the representationof the stereo foreground object in the stereo foreground object imagebased on camera motion representing a change in position of a camerabetween capturing the selected frame and capturing the current frame. 5.The method of claim 4, wherein transforming the stereo foreground objectimages comprises: enlarging the representation of the stereo foregroundobject in the stereo foreground object image in response to the camerabeing closer to a position of the foreground object when the selectedframe was captured; and reducing in size the representation of theforeground object in the foreground object image in response to thecamera being further from the position of the foreground object when theselected frame was captured.
 6. The method of claim 1, whereintransforming the stereo foreground object images comprises: translatinga location of the representation of the stereo foreground object in theforeground object image based on a camera motion representing a changein planar position of a camera between capturing the selected frame andcapturing the current frame.
 7. The method of claim 6, whereintransforming the stereo foreground object images comprises translatingthe location according to a motion equal and opposite to the cameramotion.
 8. The method of claim 1, wherein transforming the stereoforeground object images comprises: applying a rotation to the stereoforeground object image based on camera motion representing a change inrotational position of a camera about the foreground object betweencapturing the selected frame and capturing the current frame.
 9. Themethod of claim 1, further comprising: detecting an overlaid stereoforeground object image exceeding an overlap threshold of at least oneother overlaid stereo foreground object image; and applying a partialtransparency to the detected overlaid stereo foreground object imageexceeding the overlap threshold.
 10. The method of claim 1, whereinperforming the foreground/background segmentations comprises applying anadaptive Gaussian Mixture Model as a predictive model.
 11. Anon-transitory computer-readable storage medium storing instructions forgenerating a composite output image from an input video having asequence of frames, the instructions when executed by a processorcausing the processor to perform steps comprising: receiving a sequenceof stereo video frames depicting a foreground object; for selectedframes in the sequence of stereo video frames, performingforeground/background segmentations to extract respective stereoforeground object images each comprising a representation of theforeground object with background pixels subtracted and storing therespective stereo foreground object images to a foreground object list,each of the stereo foreground object images having left and right imageswith a disparity between them; transforming the stereo foreground objectimages to adjust the respective disparities between the respective leftand right images based on a change between a convergence depth for therespective selected frames and a convergence depth for the currentframe; and overlaying the transformed stereo foreground object imagesonto the current frame to generate a composite output image.
 12. Thenon-transitory computer-readable storage medium of claim 11, whereintransforming the stereo foreground object images comprises: decreasingthe disparity for a given stereo foreground object image in response tothe convergence depth for the current frame being further away than aconvergence depth for a selected frame corresponding to the given stereoforeground object; and increasing the disparity for the given stereoforeground object image in response to the convergence depth for thecurrent frame being closer than the convergence depth for the selectedframe corresponding to the given stereo foreground object.
 13. Thenon-transitory computer-readable storage medium of claim 11, wherein theinstructions when executed further cause the processor to perform a stepof: setting the convergence depth for the current frame based on a depthof a current position of the foreground object in the current framerelative to a current camera position when the current frame iscaptured.
 14. The non-transitory computer-readable storage medium ofclaim 11, wherein transforming the stereo foreground object imagesfurther comprises: scaling the representation of the stereo foregroundobject in the stereo foreground object image based on camera motionrepresenting a change in position of a camera between capturing theselected frame and capturing the current frame.
 15. The non-transitorycomputer-readable storage medium of claim 14, wherein transforming thestereo foreground object images comprises: enlarging the representationof the stereo foreground object in the stereo foreground object image inresponse to the camera being closer to a position of the foregroundobject when the selected frame was captured; and reducing in size therepresentation of the foreground object in the foreground object imagein response to the camera being further from the position of theforeground object when the selected frame was captured.
 16. Thenon-transitory computer-readable storage medium of claim 11, whereintransforming the stereo foreground object images comprises: translatinga location of the representation of the stereo foreground object in theforeground object image based on a camera motion representing a changein planar position of a camera between capturing the selected frame andcapturing the current frame.
 17. The non-transitory computer-readablestorage medium of claim 16, wherein transforming the stereo foregroundobject images comprises translating the location according to a motionequal and opposite to the camera motion.
 18. The non-transitorycomputer-readable storage medium of claim 11, wherein transforming thestereo foreground object images comprises: applying a rotation to thestereo foreground object image based on camera motion representing achange in rotational position of a camera about the foreground objectbetween capturing the selected frame and capturing the current frame.19. The non-transitory computer-readable storage medium of claim 11,wherein the instructions when executed further cause the processor toperform steps including: detecting an overlaid stereo foreground objectimage exceeding an overlap threshold of at least one other overlaidstereo foreground object image; and and applying a partial transparencyto the detected overlaid stereo foreground object image exceeding theoverlap threshold.
 20. The non-transitory computer-readable storagemedium of claim 11, wherein performing the foreground/backgroundsegmentations comprises applying an adaptive Gaussian Mixture Model as apredictive model.